Update GpuIntegrationTest Docs

Add docs to GpuIntegrationTest._RestartBrowser to describe when Browser can be
None. This is a follow up to the previous review:

https://chromium-review.googlesource.com/c/chromium/src/+/1715778

Bug: 985530
Change-Id: Icd249b5e118cfc2869abb78b4a8fcd3b3088a0e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1753806Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687387}

content/test/gpu/gpu_tests/gpu_integration_test.py

@@ -157,6 +157,10 @@ class GpuIntegrationTest(
   @classmethod
   def _RestartBrowser(cls, reason):
     logging.warning('Restarting browser due to '+ reason)
+    # The Browser may be None at this point if all attempts to start it failed.
+    # This can occur if there is a consistent startup crash. For example caused
+    # by a bad combination of command-line arguments. So reset to the original
+    # options in attempt to successfully launch a browser.
     if cls.browser is None:
       cls.SetBrowserOptions(cls._original_finder_options)
       cls.StartBrowser()